Abstract:
A thorough understanding of fault seal processes is important in many practical and geological applications, which depend on subsurface flow of fluids. While the mechanisms involved in fault sealing are well known, the microscale processes involved and their relative contribution to sealing remain debatable. In particular, the extent to which diagenetic processes overprint cataclastic fault sealing has not been resolved, mainly due to the long time scales required to measure these effects. Here, we report results from a novel suite of room temperature experiments that combined continuous analysis of dissolved silica using on-line high performance liquid chromatography, with low strain rate creep loading on sandstone cores. This technique allowed changes in silica concentration during different phases of deformation to be resolved, and revealed a 7-fold increase in overall silica concentration immediately after dynamic faulting by localised cataclasis. Calculations based on these results show that the mass of dissolved silica from the resultant fault gouge increased by up to two orders of magnitude relative to that from the intact rock over the same time scale. This increase represents the first stage of the inherent diagenetic sealing capacity of the fault, presumably through localised diffusive mass transfer. Post-test microstructural studies suggest that the magnitude of diagenetic self-sealing depends on lithological and mechanical attributes of the host rock, which control fault gouge microstructure. Our experiments suggest that diagenetic processes may account for permeability reduction of up to two orders of magnitude, comparable to reductions due to cataclasis alone. Together, these two processes account for the 5-6 orders of magnitude reduction of permeability observed in natural faults and deformation bands.